The well-known process for mounting an IC chip on a circuit substrate by using junctioning bumps is exemplified not only by the tab method of effecting the connections by the thermal fusion or ultrasonic bonding by using either the bumps formed on the IC chip at the side of the bonding pads or the bumps formed at the lead fingers at the side of a chip carrier tape but also by the flip-chip method of soldering the bumps formed at the side of the IC chip and the conductor pattern of the circuit substrate.
This process for mounting the IC chip on the circuit substrate is troubled by a high cost because the tab method requires a number of steps for the means for forming the bumps at the side of the IC chip. Thus, it is a tendency to adopt the method of transferring the bumps to the leading ends of the lead fingers. This method requires not only a special jig for forming the bumps but also a transfer bonding step and is limited in its high integration. Moreover, these two methods are suffered from a mechanical weakness because the lead fingers are not supported. Since, moreover, there is no insulator between those lead fingers, it is impossible to stagger the bonding pads. Still moreover, the lead fingers are liable to be deformed by their projecting structure, and it is difficult to hold the positional precision in the X, Y and Z directions.
Since, on the other hand, the bumps are likewise formed at the side of the IC chip in case of the flip chip method, a number of steps are required to raise the cost. This cost rise is also invited by forming a dam for preventing the solder from flowing to the peripheries of the pads of the circuit substrate. At the same time, a high integration is limited.
Therefore, the structure of the IC mounting circuit substrate, in which a predetermined circuit conductor pattern is formed on one face of an insulating film and in which IC pad junctioning bumps are made of solder such that they are electrically junctioned to the desired portions of the circuit conductor pattern and project to the outside of the circuit substrate through the aforementioned insulating layer, is highly practical as the means for solving the aforementioned several problems of the prior art. In order to mount the IC chip on that IC mounting circuit substrate, there is generally adopted a process for connecting and mounting the IC chip with and on the circuit substrate by melting solder bumps through a reflow furnace such that the pads of the IC chip are positioned over the bumps of the circuit substrate.
However, the method of connecting the IC chip by the reflow means of the solder bumps is troubled by the following problems: that the IC chip is damaged by the thermal impact at the time of passing the reflow furnace; and that the IC chip is also damaged due to the difference in thermal expansion between the circuit substrate and the IC chip.